1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus.
2. Description of the Related Art
Conventionally, there are known to be optical scanning apparatuses included in image forming apparatuses which employ a method of using a polygon mirror to deflect a group of light beams emitted from a semiconductor laser that includes multiple light emitting elements (light emitting units) and irradiate a photosensitive member (photosensitive drum) with the deflected light beams. With this kind of optical scanning apparatus, there are cases where the light beams emitted from the light emitting elements form images at positions on the photosensitive member that are different in the main scanning direction. In such a case, the writing start positions in the main scanning direction of the electrostatic latent images that are to be formed by the light beams emitted from the light emitting elements need to coincide with each other in the sub-scanning direction. To achieve this, a method is known in which two light beams emitted from two specific light emitting elements are detected by an optical sensor, and the light beam emission timings of the light emitting elements are controlled based on the result of measuring the time interval between detection signals output from the sensor.
For example, Japanese Patent Laid-Open No. 2008-28509 discloses an optical scanning apparatus that scans the surface of a photosensitive member with multiple light beams by using an optical deflector to deflect light beams emitted from light emitting points in a light source including three or more light emitting points arranged linearly at a predetermined interval. The optical scanning apparatus disclosed in the patent document above measures the interval between the two scanning lines arranged the farthest from one another in the sub-scanning direction among the scanning lines corresponding to the light beams and adjusts the interval between the scanning lines in the sub-scanning direction.
However, in the case of detecting at least two light beams with the optical sensor and measuring the time interval between the detection signals output from the optical sensor as described above, there is a possibility that the light powers of the light beams will decrease due to the optical system on the optical axis from when a light beam is emitted from a light emitting element until it reaches the optical sensor. In such a case, there is a possibility that an error will occur in the measurement result for the time interval.
Here, FIG. 1B is a diagram showing a relationship between the light powers of eight light beams emitted from eight light emitting elements with respect to the main scanning direction in the case where the semiconductor laser includes eight light emitting elements (LD1 to LD8). Note that with respect to the main scanning direction, the position at which the optical sensor is arranged (beam detection position) is shown as the reference (0 mm), and the light beam corresponding to LD1 is shown as the light beam that precedes the other light beams in the main scanning direction. Also, FIG. 1A is a diagram showing a relationship between the delay time for a signal output from the optical sensor and the light power of a light beam incident on the optical sensor.
As shown in FIG. 1B, the light beams corresponding to LD4 to LD8 can be detected by the optical sensor at 100% of their light powers (normalized using the maximum value) at the beam detection position. On the other hand, the light beam corresponding to LD1 can only be detected by the optical sensor at around 60% of its light power at the beam detection position. This is because a portion of the light beam (optical flux) corresponding to LD1 is lost due to the light beam corresponding to LD1 being incident on the end portion of the reflecting surface of the polygon mirror that is arranged on the optical axis and deflects the light beam. In the case where the light power of the light beam incident on the optical sensor decreases from 100% to 60% in this way, the delay time for the output signal of the optical sensor is extended by about 0.05 μs, as shown in FIG. 1A.
Accordingly, in a case where the light powers of light beams, which are used to measure the time interval between detection signals output from the optical sensor, at the time of being incident on the optical sensor decreases due to the optical system as described above, variation occurs in the difference in the delay time between the light beams when the detection signals are output from the optical sensor. As a result, there is a possibility that an error will occur in the measurement result for the time interval between the detection signals output from the optical sensor, and the correction accuracy for the light beam emission timings will deteriorate.